Method for Producing a High Strength Coated Steel Sheet having Improved Strength, Ductility and Formability

ABSTRACT

A method is for producing a high strength coated steel sheet having a yield stress YS&gt;800 MPa, a tensile strength TS&gt;1180 MPa, and improved formability and ductility. The steel contains: 15%≤C≤0.25%, 1.2%≤Si≤1.8%, 2%≤Mn≤2.4%, 0.1%≤Cr≤0.25%, Al≤0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature higher than Ac3 and lower than 1000° C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 250° C. and 350° C., to obtain a structure consisting of at least 60% of martensite and a sufficient austenite content such that the final structure contains 3% to 15% of residual austenite and 85% to 97% of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 430° C. and 480° C. and maintained at this temperature for a partitioning time Pt between 10 s and 90 s, then hot dip coated and cooled to the room temperature.

This is a continuation of U.S. application Ser. No. 15/322,829 which has a filing date of Dec. 29, 2016 and which is a national stage of PCT/M2015/055039 filed Jul. 3, 2015 which claims priority to PCTIB2014/002275 filed Jul. 3, 2014, the entire disclosures of which are hereby incorporated by reference herein.

The present invention relates to a method for producing a high strength coated steel sheet having improved strength, ductility and formability and to the sheets obtained with the method.

BACKGROUND

To manufacture various equipments such as parts of body structural members and body panels for automotive vehicles, it is usual to use galvanized or galvannealed sheets made of DP (dual phase) steels or TRIP (transformation induced plasticity) steels.

For example, such steels which include a martensitic structure and/or some retained austenite and which contain about 0.2% of C, about 2% of Mn, about 1.7% of Si have a yield strength of about 750 MPa, a tensile strength of about 980 MPa, a total elongation of more than 8%. These sheets are produced on continuous annealing line by quenching from an annealing temperature higher than Ac₃ transformation point, down to an overaging temperature above Ms transformation point and maintaining the sheet at the temperature for a given time. Then the sheet is galvanized or galvannealed.

To reduce the weight of the automotive so as to improve their fuel efficiency in view of the global environmental conservation, it is desirable to have sheets having improved yield and tensile strength. But such sheets must also have a good ductility and a good formability and more specifically a good stretch flangeability.

In this respect, it is desirable to have sheets having a yield strength YS of at least 800 MPa, a tensile strength TS of about 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER according to ISO standard 16630:2009 of more than 25%. It must be emphasized that, due to differences in the methods of measure, the values of hole expansion ration HER according to the ISO standard are very different and not comparable to the values of the hole expansion ratio λ according to the JFS T 1001 (Japan Iron and Steel Federation standard).

SUMMARY

The present disclosure provides such sheet and a method to produce it.

A method is provided for producing a high strength coated steel sheet having an improved ductility and an improved formability, the sheet having a yield strength YS of at least 800 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%, by heat treating and coating a steel sheet wherein the chemical composition of the steel contains in weight %:

-   -   0.15%≤C≤0.25%     -   1.2%≤Si≤1.8%     -   2%≤Mn≤2.4%     -   0.1%≤Cr≤0.25%     -   Al≤0.5%         the remainder being Fe and unavoidable impurities.

The heat treatment and the coating comprise the following steps:

-   -   annealing the sheet at an annealing temperature TA higher than         Ac3 but less than 1000° C. for a time of more than 30 s,     -   quenching the sheet by cooling it down to a quenching         temperature QT between 250° C. and 350° C., at a cooling speed         sufficient to obtain a structure consisting of martensite and         austenite just after quenching, the martensite content being at         least 60% and the austenite content being such that the final         structure contains 3% to 15% of residual austenite and 85% to         97% of martensite and bainite without ferrite,     -   heating the sheet up to a partitioning temperature PT between         430° C. and 480° C. and maintaining the sheet at this         temperature for a partitioning time Pt between 10 s and 90 s,     -   hot dip coatings the sheet and,     -   cooling the sheet down to the room temperature.

The chemical composition of the steel can, optionally, satisfy one or more of the following conditions: 0.17%≤C≤0.21%, 1.3%≤Si≤1.6% and 2.1%≤Mn≤2.3%.

In a particular embodiment, the hot dip coating step is a galvanizing step.

In another particular embodiment, the hot dip coating step is a galvannealing step with an alloying temperature TGA between 480° C. and 510° C.

Preferably, the cooling speed during the quenching is of at least 20° C./s, preferably at least 30° C./s.

Preferably, the method further comprises, after the sheet is quenched to the quenching temperature and before heating the sheet up to the partitioning temperature PT, a step of holding the sheet at the quenching temperature for a holding time comprised between 2 s and 8 s, preferably between 3 s and 7 s. In a particular embodiment, the at least one coated face is galvanized.

A coated steel sheet is also provided, the chemical composition of the steel containing in weight %:

-   -   0.15%≤C≤0.25%     -   1.2%≤Si≤1.8%     -   2%≤Mn≤2.4%     -   0.1≤Cr≤0.25%     -   Al≤0.5%         the remainder being Fe and unavoidable impurities. The structure         of the steel consists of 3% to 15% of residual austenite and 85%         to 97% of martensite and bainite, without ferrite. A least one         face of the sheet comprises a metallic coating. The sheet has a         yield strength of at least 800 MPa, a tensile strength of at         least 1180 MPa, a total elongation of at least 14% and a hole         expansion ratio HER of at least 30%.

The chemical composition of the steel can, optionally, satisfy one or more of the following conditions: 0.17%≤C≤0.21%, 1.3%≤Si≤1.6% and 2.1%≤Mn≤2.3%.

In another particular embodiment, the at least one coated face is galvannealed.

Preferably, the C content in the retained austenite is of at least 0.9%, still preferably of at least 1.0%, and up to 1.6%.

The average austenitic grain size, i.e. the average grain size of the retained austenite, is preferably of 5 μm or less.

The average size of the grains or blocks of martensite and bainite is preferably of 10 μm or less.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in details but without introducing limitations and illustrated by the figure which is a micrograph of example 8.

DETAILED DESCRIPTION

According to the present disclosure, the sheet is obtained by hot rolling and optionally cold rolling of a semi product which chemical composition contains, in weight %:

-   -   0.15% to 0.25%, and preferably more than 0.17% preferably less         than 0.21% of carbon for ensuring a satisfactory strength and         improving the stability of the retained austenite which is         necessary to obtain a sufficient elongation. If carbon content         is too high, the hot rolled sheet is too hard to cold roll and         the weldability is insufficient.     -   1.2% to 1.8%, preferably more than 1.3% and less than 1.6% of         silicon in order to stabilize the austenite, to provide a solid         solution strengthening and to delay the formation of carbides         during overaging without formation of silicon oxides at the         surface of the sheet which is detrimental to coatability.     -   2% to 2.4% and preferably more than 2.1% and preferably less         than 2.3% of manganese to have a sufficient hardenability in         order to obtain a structure containing at least 65% of         martensite, tensile strength of more than 1150 MPa and to avoid         having segregation issues which are detrimental for the         ductility.     -   0.1% to 0.25% of chromium to increase the hardenability and to         stabilize the retained austenitic in order to delay the         formation of bainite during overaging.     -   up to 0.5% of aluminum which is usually added to liquid steel         for the purpose of deoxidation. Preferably, the Al content is         limited to 0.05%. If the content of Al is above 0.5%, the         austenitizing temperature will be too high to reach and the         steel will become industrially difficult to process.

The remainder being iron and residual elements resulting from the steelmaking. In this respect, Ni, Mo, Cu, Nb, V, Ti, B, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, their contents are less than 0.05% for Ni, 0.02% for Mo, 0.03% for Cu, 0.007% for V, 0.0010% for B, 0.005% for S, 0.02% for P and 0.010% for N. Nb content is limited to 0.05% and Ti content is limited to 0.05% because above such values large precipitates will form and formability will decrease, making the 14% of total elongation more difficult to reach

The sheet is prepared by hot rolling and optionally cold rolling according to the methods known by those which are skilled in the art.

After rolling the sheets are pickled or cleaned then heat treated and hot dip coated.

The heat treatment which is made preferably on a combined continuous annealing and hot dip coating line comprises the steps of:

-   -   annealing the sheet at an annealing temperature TA higher than         the Ac₃ transformation point of the steel, and preferably higher         than Ac₃+15° C. i.e. higher than about 850° C., to ensure that         the structure is completely austenitic, but less than 1000° C.         in order not to coarsen too much the austenitic grains. The         sheet is maintained at the annealing temperature i.e. maintained         between TA-5° C. and TA+10° C., for a time sufficient to         homogenize the chemical composition and the structure. This time         is preferably of more than 30 s but does not need to be of more         than 300 s.     -   quenching the sheet by cooling down to a quenching temperature         QT lower than the Ms transformation point at a cooling rate         enough to avoid ferrite and bainite formation. The quenching         temperature is between 250° C. and 350° C. in order to have just         after quenching a structure consisting of martensite and         austenite. This structure contains at least 60% of martensite         and contains a sufficient amount of austenite in order to be         able to obtain a final structure i.e. after partitioning,         coating and cooling to the room temperature, containing between         3 and 15% of residual austenite and between 85 and 97% of the         sum of martensite and bainite without ferrite. Preferably, the         cooling rate is higher than or equal to 20° C./s, still         preferably higher than or equal to 30° C./s, for example of         about 50° C./s. A cooling rate higher than 30° C./s is enough.     -   reheating the sheet up to a partitioning temperature PT between         430° C. and 480° C. and preferably between 435° C. and 465° C.         For example, the partitioning temperature can be equal to the         temperature at which the sheet must be heated in order to be hot         dip coated, i.e. between 455° C. and 465° C. The reheating rate         can be high when the reheating is made by induction heater, but         that reheating rate had no apparent effect on the final         properties of the sheet. Preferably, between the quenching step         and the step of reheating the sheet to the partitioning         temperature PT, the sheet is held at the quenching temperature         for a holding time comprised between 2 s and 8 s, preferably         between 3 s and 7 s.     -   maintaining the sheet at the partitioning temperature PT for a         time Pt between 10 s and 90 s. Maintaining the sheet at the         partitioning temperature means that during partitioning the         temperature of the sheet remains between PT−20° C. and PT+20°         C.,     -   optionally, adjusting the temperature of the sheet by cooling or         heating in order to be equal to the temperature at which the         sheet has to be heated in order to be hot dip coated.     -   hot dip coating the sheet. The hot dip coating may be, for         example, galvanizing or galvannealing but all metallic hot dip         coating is possible provided that the temperatures at which the         sheet is brought to during coating remain less than 650° C. When         the sheet is galvanized, it is done with the usual conditions.         When the sheet is galvannealed, the temperature of alloying TGA         must not be too high to obtain good final mechanical properties.         This temperature is preferably between 500° and 580° C.     -   generally, after coating, the coated sheet is processed         according to the known art. In particular, the sheet is cool to         the room temperature.

This treatment allows obtaining a final structure i.e. after partitioning, coating and cooling to the room temperature, containing between 3 and 15% of residual austenite and between 85 and 97% of the sum of martensite and bainite without ferrite.

Furthermore, this treatment allows obtaining an increased C content in the retained austenite, which is of at least 0.9%, preferably even of at least 1.0%, and up to 1.6%.

Moreover, the average austenitic grain size is preferably of 5 μm or less, and the average size of the blocks of bainite or martensite is preferably of 10 μm or less.

The amount of retained austenite is for example of at least 7%.

With such treatment, coated sheets having a yield strength YS of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER according to the ISO standard 16630:2009 of at least 30% can be obtained.

As an example a sheet of 1.2 mm in thickness having the following composition: C=0.19%, Si=1.5% Mn=2.2%, Cr=0.2%, the remainder being Fe and impurities, was manufactured by hot and cold rolling. The theoretical Ms transformation point of this steel is 375° C. and the Ac₃ point is 835° C.

Samples of the sheet were heat treated by annealing, quenching and partitioning then galvanized or galvannealed, and the mechanical properties were measured.

The conditions of treatment and the properties obtained are reported at table I for the samples that were galvanized and at table II for the samples that were galvannealed. All steels have been annealed above Ac3 measured by experimental method. The sheet were held at the quenching temperature for about 3 s. The cooling speed during quenching was of about 50° C./s.

TABLE I RA C % BM grain in grain TA QT PT Pt YS TS UE TE HER RA size RA size Sample ° C. ° C. ° C. s MPa MPa % % % % μm % μm 1 870 300 400 60 1169 1265 4 4 2 870 300 460 60 1029 1182 10 15 40 7 ≤5 1.03 ≤10 3 870 300 500 60 883 1181 7 9

TABLE II RA C % BM grain in grain TA QT PT Pt TGA YS TS UE TE HER RA size RA size Sample ° C. ° C. ° C. s ° C. MPa MPa % % % % μm % μm 4 870 300 460  30* 500 1028 1194 10 15 32 7.7 ≤5 1.05 ≤10 5 870 300 460 60 500 973 1201 10 15 30 7.5 ≤5 1.03 ≤10 6 870 300 460  10* 500 1074 1203 9 14 7 870 300 460  10** 500 938 1208 9 14 8 870 300 460  60* 500 973 1201 10 15 30 9 870 300 440 30 500 1003 1201 9 14 10 870 300 460- 30 500 950 1216 10 13 480 11 870 300 480 30 500 857 1193 9 9 *reheating rate after quenching: 5° C./s **reheating rate after quenching: 20° C./s

In these tables, TA is the annealing temperature, QT the quenching temperature, PT the partitioning temperature, Pt the time of maintaining at the partitioning temperature, TGA the temperature of alloying for the sheets that are galvannealed, YS is the yield strength, Ts is the tensile strength, UE is the uniform elongation, Te is the total elongation and HER is the hole elongation ratio measured according to the ISO standard 16630:2009. RA % is the amount of retained austenite in the microstructure, RA grain size is the average austenite grain size, C % in RA is the C content in the retained austenite, and BM grain size is the average size of the grains or blocks of martensite and bainite.

Samples 1, 2, 3 which are galvanized show that in order to obtain the desired properties and more specifically the ductility properties, the partitioning temperature PT has to be near 460° C. i.e. the temperature for hot dip coating. When the partitioning temperature PT is 400° C. or below or 500° C. or above, in particular not within the range 430-480° C., the ductility is strongly reduced and is not sufficient.

Samples 4 to 9 and 11, which are galvannealed, show that a partitioning temperature of 460° C. or less yields the best results.

For example 10, the partitioning was made by heating up to 480° C. then linear cooling down to 460° C.

Examples 4 to 8 show that, with a partitioning temperature of 460° C. and a partitioning time between 10 s and 60 s, it is possible to obtain the desired properties on galvannealed sheets. These examples show also that it is preferable to have a partitioning time of less than 60 s, preferably about 30 s because with such partitioning time, the yield strength is higher than 1000 MPa while it is less than 1000 MPa when the partitioning time is 60 s. The micrograph of the figure illustrates example 8 which contains 7.5% of retained austenite and 92.5% of martensite+bainite.

Examples 10 and 11 show that, when the partitioning temperature is above 460° C., the ductility is significantly reduced.

Example 9 shows that, on the contrary, when the partitioning temperature is 440° C., i.e. lower than 460° C., the properties and in particular ductility remain good. 

What is claimed is:
 1. A method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the coated steel sheet having a yield strength YS of at least 800 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%, comprising: providing a steel sheet having a chemical composition including in weight %: 0.15%≤C≤0.25% 1.2%≤Si≤1.8% 2%≤Mn≤2.4% 0.1%≤Cr≤0.25% Al≤0.5% a remainder being Fe and unavoidable impurities, annealing the sheet at an annealing temperature TA higher than Ac3 but less than 1000° C. for a time of more than 30 s, quenching the sheet by cooling it down to a quenching temperature QT between 250° C. and 350° C., at a cooling speed sufficient to obtain a final structure consisting of martensite, bainite and austenite just after quenching, and such that the final structure consists of between 3% to 15% of retained austenite and 85% to 97% of a sum of martensite and bainite, with a martensite content of at least 60%, heating the sheet up to a partitioning temperature PT between 430° C. and 480° C. and maintaining the sheet at this temperature for a partitioning time Pt between 10 s and 90 s, hot dip coating the sheet, and cooling the sheet down to the room temperature.
 2. The method according to claim 1, wherein the chemical composition of the steel sheet includes 0.17%≤C≤0.21%.
 3. The method according to claim 1, wherein the chemical composition of the steel sheet includes 1.3%≤Si≤1.6%.
 4. The method according to claim 1, wherein the chemical composition of the steel sheet includes 2.1%≤Mn≤2.3%.
 5. The method according to claim 1, wherein the hot dip coating step is a galvanizing step.
 6. The method according to claim 1, wherein the hot dip coating step is a galvannealing step with an alloying temperature TGA between 480° C. and 510° C.
 7. The method according to claim 1, wherein the cooling speed during quenching is at least 20° C./s.
 8. The method according to claim 7, wherein the cooling speed during quenching is at least 30° C./s.
 9. The method according to claim 1, further comprising, after the step of quenching and before the step of heating, a step of holding the steel sheet at the quenching temperature QT for a holding time between 2 s and 8 s
 10. The method according to claim 9, wherein the holding time at the quenching temperature QT is between 3 s and 7 s.
 11. The method according to claim 1, wherein the C content in the retained austenite is of at least 0.9%.
 12. The method according to claim 11, wherein the C content in the retained austenite is of at least 1.0%.
 13. The method according to claim 1, wherein the average grain size of the retained austenite is of 5 μm or less.
 14. The method according to claim 1, wherein the final structure includes at least 7% of retained austenite. 